TLR4 and mastitis: TLR2 and TLR4 are particularly important in mammary gland immune defense because they are pathogen-associated molecular pattern ligands (PAMPs) that cause mastitis linked to Gram-positive (peptidoglycan, LTA) and Gram-negative (lipopolysaccharide, LPS) pathogens, including S. aureus, S. uberis and E. coli [14,16,20].